At present, mobile communication systems such as cellular telephone systems and wireless LANs (Local Area Network) are widely deployed. Mostly, cellular-type mobile communication systems are employed. In a cellular type, which is one of the communication forms, a plurality of base stations that serve a set of cells forming a service area are installed, and communications are controlled by these base stations cooperating with each other. A mobile station accesses a cell with good communication quality out of cells that are accessible from where it is now, and performs radio communication.
In order to select an optimal cell, the mobile station performs a cell search. In the cell search, the mobile station establishes synchronization with each radio signal from base stations, and obtains information required for selecting a cell. A base station continuously transmits a signal on a synchronization channel (SCH) in a given frequency band. For use on a synchronization channel, a scrambling sequence is previously assigned to each cell. The mobile station detects a correlation between a signal on a synchronization channel received from a base station and each candidate of scrambling sequence, to thereby specify a scrambling sequence used in the cell to which the mobile station currently belongs. Through this process, the mobile station recognizes the cell to which it currently belongs, and detects the timing of receiving radio signals. In this connection, at least different scrambling sequences are assigned to adjacent cells. Therefore, even when the mobile station belongs to a plurality of cells, it can distinguish and recognize them (for example, refer to the following two literatures). It is noted that, in general, the band for a synchronization channel is set in the center of a transmission band.    3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)”, 3GPP TS36.300, 2007-06, V8.1.0    3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, 3GPP TS36.211, 2007-09, V2.0.0.
By the way, in the conventional radio communication systems, macrocells with a radius of 1 to several km are mainly provided. On the other hand, the recent trend is to provide many cells with a small radius for supporting macrocells in order to realize high-rate and stable data communication. Especially, a cell with as small a radius as several m is called a femtocell. By introducing such a femtocell in a house or office, a mobile station in such a place may access the femtocell, which reduces the need of accessing the macrocells. This leads to a reduction in loads imposed on the base stations serving the macrocells, and a further improvement in communication quality.
To provide a femtocell over a macrocell, there is a method of setting an offset to the transmission band of the femtocell so as to separate the transmission band of the macrocell and the transmission band of the femtocell in the macrocell (for example, refer to 3rd Generation Partnership Project, “Measurement of home & private eNBs”, 3GPP TSG-RAN WG2 Meeting #59 R2-073307, 2007-08). In addition, there is another method of allocating a femtocell a narrower bandwidth than a macrocell in order to reduce interference between them (for example, refer to 3rd Generation Partnership Project, “Spectrum Arrangement to enable Co-channel deployment of Home NodeBs”, 3GPP TSG-RAN WG4 Meeting ##44 R4-071494, 2007-08).
It is expected that a plurality of relatively small cells, like femtocells, will be provided in a large cell.
In doing so, if a large cell and small cells are allocated a completely same transmission band (transmission frequency band) for synchronization channels, a mobile station takes a time to determine a desired cell. This is a problem.
Then consider providing femtocells that are not open to use by all mobile stations but are accessible from only admitted mobile stations, for example. Though these admitted mobile stations may attempt to search a desired femtocell as fast as possible, the use of the same transmission band for synchronization channels in common among the femtocells and the large cell causes the mobile stations to individually receive synchronization channels that are transmitted in the same band, individually establish synchronization to obtain information such as broadcast information, and then determine whether a cell being searched is a desired femtocell (or whether the mobile station has access rights to the cell). Therefore, it also takes a time.
In short, it requires a considerable time to determine a desired cell in an environment where the large cell and the plurality of small cells coexist.